Choosing Timber

When choosing timber for a job, then it is important to ensure that a suitable species is selected. There are many things that should be considered to ensure that the life of the timber. This is just as important for home owners as it is for Architects and Builders

The following list of 3 basic questions can be used as a basic checklist to help make a decision on the correct timber selection.

1. Is the timber structural or non-structural?

Timber that is structural should conform to the Australian Standard AS 1684

2. Is the timber for internal or external use?

Timber that is to be used for external purposes should be chosen in terms of its decay durability.

All timber species are assigned to a durability class or “Hazard Class” of 1 to 4. Class 1 durability timbers are the most durable and hence the most suitable for external use, while class 4 timbers are the least suitable.

Durability Class

Probable in-ground life expectancy (Years)

Probable above ground life expectancy (Years)

1

> 25

> 40

2

15-25

15-40

3

5-15

7-15

4

0-5

0-7

Low durability timbers such as radiata pine (natural durability class 4), can be treated to increase their resistance to decay and termite attack with CCA or LOSP preservatives. The treatment applied to softwoods falls into Hazard Class ratings as per the Australian Standard 1604.

AS 1604 also states that treated timber be stamped to identify

the level of treatment

the chemical treatment applied

the company that treated the timber

The primary treatment carries a 25 year guarantee against failure PROVIDED that all end cuts are treated with an approved “End Seal” application which is then sealed with a suitable paint primer. Currently the only available product to achieve this are Ecoseal (for H3/H2) and EnSeal (H2 only) by Tanalized.

The following table lists the recommended treatment ratings for specific uses.

Hazard Class

Exposure

Typical Uses

1

Interior above ground

Framing, flooring, furniture

2

Interior above ground

Framing, flooring, furniture

3

Exterior above ground

Decking, weatherboards, windows

4

Exterior in ground

Fence posts, stumps, landscaping

5

Exterior in ground, contact with or in fresh water

Retaining walls, stumps and poles, cooling tower fill

6

Marine water

Boat hulls, marine piles, pier structural timbers

3. Is the timber to be used in a high fire risk region?

Timbers that are to be used in construction of buildings in high fire risk regions need to conform to the local building codes. The Australian Standards that deal with the requirements of timbers in high fire risk zones are: